CS 654 Digital Image Analysis Lecture 17 Image

CS 654: Digital Image Analysis Lecture 17: Image Enhancement

Recap of Lecture 16 • Mask processing • Spatial filtering • Averaging • Differentiation • Correlation • Convolution

Outline of Lecture 17 • Image enhancement • Dynamic range • Point processing • Contrast stretching • Intensity level slicing

Introduction • Improve the quality of an image as perceived by human being • Can be performed at both spatial and frequency domain • Different types of enhancement • Noise suppression • Increase contrast • Sharpen or smoothen image feature • Emphasise • Display and analysis

Common techniques Image enhancement Point processing • • Contrast stretching Noise clipping Window slicing Histogram modeling Spatial operation • • Transform operation Pseudocoloring Noise smoothing • Linear filter • Root filter Median filtering Unsharp masking • Homomorphic filter Filtering

Dynamic Range • The ratio between the maximum and minimum measurable light intensities • In the real world, one never encounters true white or black Image: cambridgeincolour

ILLUMINANCE & REFLECTIVITY • Both contribute to the dynamic range of a scene • Illuminance to specify only incident light • Vast variation possible for incident light

Dynamic Range of Camera Cavity Array Black Level (Limited by Noise) Light Cavities White Level (Saturated) Darker White Level (Low Capacity) Image: cambridgeincolour

Images: Gonzalez & Woods, 3 rd edition Human eye Weber ratio as a function of intensity HDR Images, Posterization

Images: Gonzalez & Woods, 3 rd edition Intensity transformation Point processing

Images: Gonzalez & Woods, 3 rd edition Point processing 255 0 255 Contrast stretching function 0 255 Thresholding function

Images: Gonzalez & Woods, 3 rd edition Intensity transformation: Negative Transformation function

Images: Gonzalez & Woods, 3 rd edition Intensity transformation: Log-transform Transformation function • Maps narrow low intensity to wider range of intensity • Compresses the dynamic range of images

Images: Gonzalez & Woods, 3 rd edition Intensity transformation: Gamma Transformation function • Fractional value maps narrow low intensity to wider range of intensity • Higher value maps narrow low intensity to wider range of intensity

Images: Gonzalez & Woods, 3 rd edition Gamma correction: Display Original image Gamma corrected image Gamma correction Input image as viewed in monitor Gamma corrected image as viewed in monitor

Images: Gonzalez & Woods, 3 rd edition Gamma correction; Dark to light Original image 0. 6 0. 4 0. 3

Images: Gonzalez & Woods, 3 rd edition Gamma correction: light to dark 3. 0 4. 0 5. 0

Images: Gonzalez & Woods, 3 rd edition Contrast stretching Spans the full range of intensity level Linear Thresholding

Images: Gonzalez & Woods, 3 rd edition Contrast stretching Input image Contrast stretched image Thresholded image

Gray-level slicing • Highlight a specific range of gray values With background Without background

Example Input image Without background With background

Bit plane slicing Bit plane representation of 8 bit images 1 2 3 4 5 6 7 8

Reconstruction of images 8+7+6+5 8+7+6

Thank you Next lecture: Histogram Processing